This invention relates to a method and apparatus for determining the density of bodies by means of penetrating rays wherein at least one radiation source means emits rays of different wavelengths, and at least one radiation measuring means forms electric signal values corresponding to the intensities which again issue from the body. Apparatus of this type is known, for instance, from the publication "Medical Application of the Compton Effect" by K. H. Reiss and B. Steinle in "Siemens-Forschungs -- und Entwicklungsberichte", Vol. 2 (1973), No. 1, pages 16 through 25.
The simple and impressive thought of using the Compton effect for determining the (electron) density in the interior of bodies, and therefore also in living organisms, has been realized in a medically significant fashion only in exceptional instances up to the present time. This is, for example, the determination of the absolute density of the lungs. Only in this instance has it been possible up to the present time, by employing a simple model, to take into consideration with sufficient precision the losses from the radiation source to the scattering location and from the scattering location to the detector; i.e., the absorption of the radiation (compare the above-cited reference). In the case of randomly shaped bodies, such as bones, for example, the conditions are already so complicated that the correction computations become problematical in themselves. Even in the case of the lungs, the back-scattering method has, until now, been limited to a depth of approximately eleven centimeters (11 cm) in the object. Numerous attempts to use measurements of the Compton-scattering of X-ray energy or gamma rays for determining the absolute density in the interior of bodies have not led to any wider application. The reason for this would essentially be a lack of radiation sources with suitable energy, that is, isotopes with suitable radiation, life and activity.
Nevertheless, there is still a great need for methods enabling one to externally determine the density of interior regions of bodies, such as in the spinal column, for example, without being disturbed by obstacles such as the intestinal vesicles. The liver would also be a medically interesting object for density determination because it is accessible only with difficulty to normal X-ray investigation. For the lungs, a method is desirable for determining the local ventilation of defined zones. It is also desirable, if possible, to be able to obtain the result in the form of a section, even if a rough section, of all parts of interest, such as is disclosed in the U.S. Letters Pat. No. 2,670,401, which, however, has not yet been realized on account of a lack of suitable radiating systems. For radiation therapy with ionizing rays there has also long been the need of being able to determine in a simple manner the density distribution in the interior of a body.